Biaxially oriented heat-shrinkable films of the monolayer type are used in the packaging of poultry. Biaxially oriented heat-shrinkable films of the multilayer type having an oxygen barrier core layer are used in the packaging of processed meat and fresh red meat. As generally understood, a "heat-shrinkable" film tends to return to its original unstretched (unextended) dimension when heated to its softening point. The terms "orientation" or "oriented" are used to describe the manufacture of heat-shrinkable films, wherein resin material is heated above its flow or melting point and extruded through a die into either tubular or sheet form. After cooling, the relatively thick extrudate is reheated to a temperature range suitable to orient or align the crystallites and/or molecules of the material. The orientation temperature range for a given material or materials is understood by those skilled in the art to be in a range which revises the inter-molecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to improve certain mechanical properties of the film such as shrink. When the stretching force is applied in one direction, uniaxial orientation results. When the stretching force is simultaneously applied in two directions, biaxial orientation results.
A commercially used thermoplastic material in the manufacture of biaxially oriented heat-shrinkable films is ethylene vinyl acetate (EVA). This material in the thin film form (e.g. 2.5 mils) is characterized by high shrink properties, for example at least 35% shrink in both the machine (MD) and transverse directions (TD).
Biaxially oriented heat-shrinkable films containing EVA are commonly produced by extruding a primary tube, cooling, and then reheating and expanding the primary tube both longitudinally and transversely by means of different nip roll speeds in the longitudinal direction and air inflation in the transverse direction. One such process is described in the U.S. Pat. No, 3,456,044. This two-step process is often referred to as the double-bubble or the trapped bubble process. When a multilayer film is desired the multiple layers may be coextruded, as for example with an oxygen-barrier core layer and first and second outer layers on each side of the core layer to form the aforementioned primary tube. As for example described in Canadian Patent No. 982,923, these outer layers may comprise EVA and the core layer may comprise a vinylidene chloride copolymer with a comonomer such as vinyl chloride or methyl acrylate. Another commonly used oxygen barrier material is ethylene vinyl alcohol, i.e. EVOH. Instead of coextrusion the primary tube may be formed by coating lamination, wherein a first outer layer is extruded and thereafter the core and second outer tubular layers are sequentially coated onto the outer surfaces of the first tubular layer and the core layer. As another alternative, the first outer and core outer layers may themselves be coextruded, and the second outer layer thereafter coated onto the outside surface of the core layer. Coating lamination procedures are described in U.S. Pat. No. 3,741,253. As still another alternative, a multiple layer film may be formed as a sheet by the well-known slot casting procedure, and then biaxially oriented into a heat-shrinkable film, for example by tentering.
One limitation of EVA-based biaxially oriented heat-shrinkable film is that the plastic orientation strength of EVA films is relatively low compared with certain other thermoplastic materials as for example polyethylene. Because of its molecular structure, EVA has relatively low strength in the primary tube form when reheated to orientation temperatures as for example 68.degree.-84.degree. C. As a result, EVA based biaxially oriented heat-shrinkable film is subject to bubble breaks and process interruptions, which can result in a relatively high waste rate.
Certain polyethylenes may be used to manufacture biaxially oriented heat-shrinkable films, and their plastic orientation strength is substantially higher than EVA. Unfortunately their heat shrink properties are inferior to EVA. By far the best heat shrink properties in the polyethylene family are achieved with very low density polyethylene (VLDPE). Although the heat shrink properties of VLDPE approach those of EVA, there is a significant difference and loss of shrink if VLDPE is substituted for EVA in the production of biaxially oriented heat-shrinkable films to achieve higher plastic orientation strength. Moreover it has been determined that the shrink and plastic orientation properties of EVA and VLDPE blends are approximately linear. That is, the shrink and plastic orientation properties of biaxially oriented heat-shrinkable films comprising 50% EVA-50% VLDPE are approximately midway between those of the pure components.
In the aforereferenced Wilhoit parent application, a biaxially oriented heat-shrinkable film is described and claimed, containing EVA and VLDPE or LLDPE (linear low density polyethylene) and having shrink properties similar to those of a pure EVA film, but also the improved plastic orientation properties of a pure VLDPE film. Use of EVA and VLDPE in such film is desirable because both are available from several manufacturers as relatively low cost resins. This Wilhoit film comprises a blend of a polyethylene member selected from the group consisting of VLDPE and LLDPE or a mixture thereof, ethylene alpha-olefin plastomer copolymer of density below about 0.90 g/cm.sup.3 and EVA as major components. The term "ethylene alpha-olefin plastomer" is described further hereinbelow. As between the polyethylene and EVA, polyethylene comprises between about 40 and about 65% of the total. The ethylene alpha-olefin plastomer copolymer comprises between about 10 and about 20 wt. % of the total weight of the three component blend.
Notwithstanding the improvement in the art afforded by this Wilhoit invention, even further improvements are desired. For example, in certain end uses such as packaging of processed meats, there is a desire for thermoplastic films with higher biaxial shrink level than achievable with pure EVA. Another need for processed meat packaging is a biaxially oriented heat-shrinkable film with improved optical properties, i.e. high gloss and lower haze. A further desire is to provide such a thermoplastic film with improved hot water seal strength.
Two component blends of relatively high concentration VLDPE and relatively low EVA concentration provide reasonably good shrink and very good hot water seal strength, but relatively poor optical properties for processed meat packaging. Two component blends of relatively high EVA concentration and relatively low plastomer concentration (of the type employed in the Wilhoit invention) likewise do not provide particularly good optical properties. Moreover, three component blends where each component is present in substantial concentration in general do not provide good optical properties. This is typically due to compatibility differences between the three components.
U.S. Pat. No. 5,1272,016 discloses a biaxially oriented heat shrinkable multilayer stretch film, useful as a trayed poultry overwrap, comprising two outer layers and a core layer. The outer layers each comprise a blend of 20-35 wt. % ethylene alpha-olefin plastomer and 65-80 wt. % VLDPE of density between 0,912 and 0,914 g/cm.sup.3. These films are suitable for the intended application, but in general do not provide sufficiently high gloss % for processed meat packaging, or the higher shrink % desired for the same end use. In addition their hot water seal strength would not be sufficiently high for processed meat packaging.
An object of this invention is to provide a biaxially oriented heat-shrinkable film of the EVA-VLDPE type, having higher biaxial heat shrink, better optical properties and improved hot water seal strength than heretofore achieved.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.
As will be explained and demonstrated hereinafter, the biaxially oriented heat-shrinkable film of the present invention provides all of the advantages of the aforedescribed Wilhoit invention, but in addition provides at least three additional improvements over the EVA-VLDPE blend-type film, i.e. higher shrink, better optical properties and better hot water seal strength.
It has been unexpectedly discovered that if VLDPE, EVA and ethylene alpha-olefin plastomer are blended in selected concentration ranges, each of these three improvements may be realized. This is particularly surprising because each of the aforementioned constituents is present in a substantial concentration, yet the resulting film has superior optical properties to two component blends of the same constituents, along with improved shrink and hot water seal strength.
More particularly, one aspect of this invention relates to a biaxially oriented heat-shrinkable film comprising a three component blend of very low density polyethylene (VLDPE), ethylene vinyl acetate (EVA) and ethylene alpha-olefin plastomer copolymer of density below about 0.90 g/cm.sup.3 as major components. The VLDPE comprises between about 24 and about 38 wt. %, the EVA comprises between about 28 and about 40 wt. %, and the plastomer comprises between about 33 and about 43 wt. % of the three component blend.
The aforedescribed film may be a monolayer wherein the three component blend comprises the entire film. Alternatively this film may comprise one or more layers of a multilayer film. By way of example, the three component blend may comprise either or both the first and second outer layers on either side of a three layer film wherein the core layer between these layers is an oxygen barrier material.
Another aspect of this invention relates to a method for manufacturing a biaxially oriented heat-shrinkable plastic film wherein a primary tube is extruded, cooled, reheated and stretched in the machine direction and inflated in the transverse direction by internal gas to form a bubble, then collapsed and withdrawn as biaxially oriented heat shrinkable film. The improvement comprises forming the primary tube from a three component blend of major constituents comprising: VLDPE, EVA and ethylene alpha-olefin plastomer copolymer of density below about 0.90 g/cm.sup.3. The VLDPE comprises between about 24 and about 38 wt. %, the EVA comprises between about 28 and about 40 wt. %, and the plastomer comprises between about 33 and about 45 wt. % of the three component blend.